Measuring instruments which are embodied for continuous tracking of a target point and for determining a position of this point in terms of coordinates can, in general, particularly in the context of industrial measuring, be subsumed by the term laser tracker. Here, a target point can be represented by a retroreflecting unit (e.g. a cube prism), which is sighted by an optical measurement beam of the measuring device, in particular by a laser beam. The laser beam is reflected back to the measuring instrument in parallel, wherein the reflected beam is acquired by an acquisition unit of the device. In the process, an emission or reception direction of the beam is established, for example by means of sensors for measuring the angle, which are assigned to a deflection mirror or a sighting unit of the system. Moreover, a distance from the measuring instrument to the target point is established with the acquisition of the beam, for example by means of a time-of-flight or phase-difference measurement or by means of the Fizeau principle.
Moreover, in relatively modern tracker systems, an offset of the received measurement laser beam from a so-called servo control point on a sensor is established—increasingly in a standardized manner. By means of this measurable offset, it is possible to determine a difference in the position between the center of a retroreflector and the point of incidence of the laser beam on the reflector, and the alignment of the laser beam can be corrected or updated as a function of this deviation in such a way that the offset on the sensor is reduced, in particular becomes “zero”, and therefore the beam is aligned in the direction of the reflector center. By updating the laser emission direction, there can be a continuous target tracking of the target point and the distance and position of the target point can be determined continuously relative to the measuring instrument. Here, the updating can be realized by means of a change in the alignment of the deflection mirror, which is movable in a motor-driven manner and provided for deflecting the laser beam, and/or by swiveling the sighting unit which includes the beam-guiding laser optics.
Coupling of the laser beam to the reflector must precede the above-described target tracking. To this end, an acquisition unit with a position-sensitive sensor and a comparatively large field of view can additionally be arranged on the tracker. Moreover, illumination means, by means of which the target or the reflector is illuminated, in particular by means of a defined wavelength differing from the wavelength of the distance measuring means, are additionally integrated into generic instruments. In this context, the sensor can be embodied to be sensitive to a range about this specific wavelength in order, for example, to reduce or completely avoid stray light influences. The illumination means can be used to illuminate the target and the camera can be used to acquire an image of the target with an illuminated reflector. By imaging the specific (wavelength-specific) reflection at the sensor, the reflection positions in the image can be determined in a resolved manner and it is therefore possible to determine an angle relative to the acquisition direction of the camera and a direction to the target or reflector. An embodiment of a laser tracker with such a target-seeking unit is known from e.g. WO 2010/148525 A1. As a function of the directional information derivable thus, it is possible to modify the alignment of the measurement laser beam in such a way that a distance between the laser beam and the reflector, to which the laser beam is intended to be coupled, is reduced.
For the purposes of measuring the distance, laser trackers from the prior art include at least one distance measuring device, wherein the latter can e.g. be embodied as an interferometer. Since such distance measuring units are only able to measure relative changes in the distance, so-called absolute distance measuring devices are installed in current laser trackers in addition to interferometers. By way of example, such a combination of measurement means for determining the distance is known from the product AT901 from Leica Geosystems AG. Furthermore, a combination of an absolute distance measuring device and an interferometer for determining the distance by means of an HeNe-laser is known e.g. from WO 2007/079600 A1.
Laser trackers according to the prior art can additionally be configured with an optical image acquisition unit with a two-dimensional, light-sensitive array, e.g. a CCD or CID camera or a camera based on a CMOS array, or with a pixel-array sensor and with an image processing unit. Here, in particular, the laser tracker and the camera can be assembled on one another in such a way that their positions relative to one another are unchangeable. By way of example, the camera is rotatable together with the laser tracker about the substantially perpendicular axis of the latter, but can be swiveled up and down independently of the laser tracker and is therefore arranged separately from, in particular, the optics of the laser beam. Furthermore, the camera can be embodied to be swivelable about one axis only—for example as a function of the respective application. In alternative embodiments, the camera can be installed integrally together with the laser optics in a common housing.
By acquiring and evaluating an image—by means of the image acquisition and image processing unit—of a so-called auxiliary measurement instrument or auxiliary measurement object with markings, the relative locations thereof with respect to one another of which are known, it is possible to deduce the orientation in space of the instrument and of an object (e.g. a probe) arranged on the auxiliary measurement instrument. Furthermore, together with the determined spatial position of the target point, it is possible to precisely determine the position and orientation of the object in space absolutely and/or relative to the laser tracker (6DoF-determination: determining six degrees of freedom).
Such auxiliary measurement instruments can be embodied as so-called contact sensing tools which, with the contact point thereof, are positioned on a point of the target object. The contact sensing tool includes markings, e.g. points of light, and a reflector, which represents a target point on the contact sensing tool and which can be targeted by the laser beam of the tracker, wherein the positions of the markings and of the reflector relative to the contact point of the contact sensing tool are known precisely. By way of example, in a manner known per se to a person skilled in the art, the auxiliary measurement instrument can also be a manually held scanner, equipped for the distance measurement, for contactless surface surveying, wherein the direction and position of the scanner measurement beam used for measuring the distance relative to the light points and the reflectors arranged on the scanner are known precisely. By way of example, such a scanner is described in EP 0 553 266.
For a reliable determination of the orientation of an object by means of a laser tracker, an in-focus (focused) image and, preferably, a known scale for the image of the object, e.g. on a camera, are advantageous. As a result thereof, an image acquirable therefrom can be evaluated quickly by means of the image processing in the case of a known ratio which is optimized for an evaluation.
Image processing systems, typically with fixed focus objectives, constitute the prior art relevant hereto. In the field of coordinate measuring machines (CMM), use is sometimes made of zoom objectives for restricted object distances. For the purposes of industrial measurement (using laser trackers), Leica Geosystems AG offers the “T-Cam” product, which can be used in a measurement range from 1.5 m to 15 m by means of a solution based on a progressive lens/zoom objective. By combining an adjustable magnification and a likewise adjustable focusing, this can always be used to image a target object (in a comparatively large manner) within this measurement range with a fixed image scale, wherein, at the same time, an in-focus image of the markings is generated. The advantage offered thereby in relation to other solutions is that, on the basis of the image which can be generated thereby, it is possible to reliably determine (as a result of the known and ideal size of the imaged object or the markings) very precisely three rotational degrees of freedom of an object to be registered, e.g. an auxiliary measurement object with known positioning of markings. As a result, this system offers increased measurement accuracy compared to alternative measurement systems of this type.
However, a disadvantage of this progressive lens optics consists of the limited measurement range which, at least in part, consists of the structurally-based limitation of the adjustability of the focusing unit and of the magnification unit. Particularly in view of an increasingly demanded miniaturization of measurement instruments in general and of tracking systems in particular, the limitation of the measurement range, connected therewith, of installed progressive lens optics according to the prior art continues to exist or could further increase with ever smaller measurement systems, which, at the same time, may lead to increasingly disadvantageous measurement conditions.